An externally-mounted speed changing device, or rear derailleur, is mounted onto a bicycle, especially onto a sports-type road racer or mountain bike. Such rear derailleur is equipped with a base component, which is mounted on the frame; a link mechanism, one end of which is mounted on the base component; a movable component, which is mounted on the other end of the link mechanism, and which can be relatively displaced from the aforementioned base component; and a chain guide, which is mounted on the movable component so as to undulate like a see-saw. The chain guide functions to transfer the chain, so as to derail the chain upon any of the rear sprockets. The chain guide is equipped with an outer plate, which is mounted on the movable component so as to undulate like a see-saw; an inner plate, which is installed so as to face the outer plate; a guide pulley, which is mounted onto a portion on one edge of the plates, between the plates, so as to freely rotate, and which can be engaged by means of the chain; and a tension pulley, which can be engaged with the aforementioned chain, and which is mounted onto a portion on the other edge of the aforementioned plates, between the plates, so as to freely rotate.
With respect to the rear derailleur structured according to the above-stated manner, a rear derailleur in which the guide pulley is mounted between the plates so as to freely move in the axial direction is well known. See Official Gazette for Kokai Utility Model Application No.:1990-10075. In the case in which the guide pulley is mounted so as to freely move in the axial direction, even when the guide pulley does not match the center of the rear sprocket due to a failed adjustment, the guide pulley is shifted to the appropriate position by means of the chain's tension. Thus, even when the derailleur is not adjusted appropriately, noise generated from the chain when it comes into contact with the rear sprocket can be prevented, and the chain may be smoothly guided.
In the traditional structure, under certain conditions, the shift of the guide pulley to the outer axial direction is occasionally conducted improperly. Such tendency is occasionally exhibited particularly on the top side of the rear sprocket, where the tension of the chain becomes weak. When the shift of the guide pulley to the outer axial direction is hindered in this manner, the guide pulley is not positioned at the center of the rear sprocket. Consequently, a noise occurs, and the chain is subject to improper guidance.
The purpose of the present invention is to render it possible to smoothly guide the chain to the rear sprocket.